I thought that it’s illegal for anyone but P.O. employees to deposit anything in a mailbox.
Did you read?
I said for the elderly couple to put money in an envelope in their mailbox once they know the person is coming with the groceries. Or in between their storm door/outside door area.
YOU can put whatever you want in your own mailbox. YOU can use it as a drop box if you arrange it with another person. Its your mailbox.
The only consideration would be to pick a time well away from regular mail delivery to avoid a confusing situation.